A method of density determination includes the introduction of a fluid sample, i.e., liquid and/or gaseous, into a U-shaped measurement cell, e.g., a glass tube capable of vibration. The fluid-filled measurement cell is excited to undamped vibration or oscillation. The density of the sample can be determined on the basis of the change in frequency and the period of the oscillation in comparison with a measurement cell filled with a standard sample. Devices for density measurement that operate by this method are known as flexural resonators.
When loading the measurement cell with a sample, it should be noted that this is done with a predefined quantity or a predefined volume of the sample and with as few bubbles as possible. Manual loading using a suitable pipette or syringe is suggested as the simplest option for filling the cell. However, this is time-consuming, so that for serial measurements in particular, the U-shaped measurement cell is connected to a pump and used as a flow-through cell and/or a sample is added and the inlet line and the outlet line are closed off by valves.
Especially in the case of laboratory instruments in which the U-shaped measurement cell is installed in a housing in a vibration-capable manner, there is the difficulty of connecting the measurement cell to a sample reservoir with a fluid-tight connection without damaging or destroying the measurement cell, which is often made of glass. Furthermore, the measurement results may be impaired because the connections of the measurement cell are also vibrating in a measurement.
Known measurement instruments have a complex adapter housing that is complicated to manufacture and have adapters which often include metal parts for manual and/or automatic loading. Metal parts in particular have the disadvantage that they cannot ensure the required thermal insulation of the measurement cell, an attempt being made to partially compensate same by mathematical correction factors or correction functions. Furthermore, the assembly and connection of the measurement cell to a sample reservoir is complex and may result in damage to the measurement cell. Furthermore, the adapters and the adapter housing also oscillate during a measurement, so that these components may compromise the measurement result on the one hand and on the other hand may be damaged or destroyed by the oscillation.
The object is thus to provide a measurement instrument for density determination using a thermally insulated measurement cell and an improved user-friendly approach for connecting the measurement cell.